An IP ("Internet Protocol") address is a unique identification number used by networked electronic devices. Each device has a unique IP address (or is behind a NAT router which does) on the Internet, which is used much like a mailing address. All communications sent to a specific device must list that device's IP address, and all communications from that device must include its source address. The IP address is used in networking on layer 3 of the OSI model. They are arbitrarily assigned to devices, at which time they are mapped to those devices' MAC addresses (OSI layer 2).
Static vs dynamic
IP addresses cannot be randomly chosen by a device; that device must be assigned an address by the network it is connecting to. This address can be assigned as either static or dynamic. Static IP addresses are permanently assigned to a specific device. This is helpful when external devices need to initiate contact with that device (such as a web server receiving a visitor). Dynamic IP addresses are issued by a DHCP server from a pool of IP addresses for a limited time, when devices request them. When a device's dynamic IP address expires, it is either renewed, or the device is assigned a new one.
IP version 4 is currently used by most network devices. However, with increasing numbers of electronic devices accessing the internet, available IPv4 addresses have been essentially depleted. IPv4 uses 32-bit numeric strings which limits it to 4,294,967,296 possible addresses. To help extend its functionality, network address translation is often used to connect many devices through only a few (often just one) public addresses. IPv4 addresses can theoretically range from
256.256.256.256, although there are reserved sets, including
192.168.x.x. The highest possible value for each octet (256) is also not available for normal usage, so 255 is the highest value which can be assigned.
IP version 6 is the replacement for the aging IPv4. This protocol theoretically offers 2^128 (340,282,366,920,938,463,463,374,607,431,768,211,456) unique addresses. However, RFC 2374 (obsoleted by RFC 3587) has limited the valid IPv6 addresses to about 2^125 at this time. Additionally, some addresses will be reserved for special purposes, and for various other reasons, not all of these theoretical addresses will but usable.
With IPv6, the amount of possible IP addresses is increased exponentially, allowing for a much greater increase in the use of mobile phones, computers, video game consoles, tablet computers, IoT devices, and even computers found within modern automobiles. IPv6 can also justify for the future growth in world population of Planet Earth.
IPv6 addresses use 64-bits, so the addresses use hexadecimal values.
Since most devices connect to a given website or service from their own unique IP address, a user can potentially be identified using it alone. Due to the possibility of shared devices, NAT systems, dynamic address reassignment, and other issues, this is not typically a guaranteed method of identification. However, it can be helpful in many situations. Whether for tracking and marketing purposes, or account security (logging in from the same IP repeatedly helps to establish trust) this can be used on its own, or paired with other identification and device "fingerprinting" methods to better establish identity.
IP addresses map to specific devices around the world, and those addresses are leased to them by their service providers.Since the service providers know the exact physical address of each customer, IP addresses can technically be traced to the device's physical location. Publicly available tools are able to get some idea of a device's location based based solely on its address. However, these tools are often limited to the knowledge of what internet service provider owns the relevant IP block, and the ISP's location. They generally only attempt to determine the state and town of origin, although these estimations can be significantly inaccurate. Sometimes the reported "geolocation" of an IP address will be the network switch nearest to the IP user, and sometimes it will only be able to report several steps back from this.
Despite these common inaccuracies, Geolocation can still be used by advertising companies (to advertise based on your approximate location), stalkers and cyber criminals. IP addresses are not at all secret, so they can be fairly easy to obtain. Any direct communication discloses the user's IP address, and sometimes it is published openly. For example, a Wikipedia® article "edit history" page that reveals the IP address of editors who did not log into user accounts.
This same basic technique can be used by the webmasters of major websites such as YouTube and Facebook to help law enforcement track down a criminal. IP address tracking can play a vital role in law enforcement, such as to track down and prosecute hackers, child molesters, stalkers, and even potential terrorists. Law enforcement agencies typically have the significant benefit of being able to obtain from ISPs (by warrant or subpoena) the exact physical address to which the IP address is registered.
However, geoloacation can also be useful for security. For example, if a user logs in to an online account from an IP that geolocates to the US, and then ten minutes later logs in from another IP which traces back to China, it is reasonable to assume that both attempts are not coming from the account owner. Sophisticated account security systems will often prompt the user(s) for additional identity verification to help prevent compromise. Additionally, some devices and systems are configured to partially or implicitly trust certain addresses (using a whitelist) or block untrusted or dangerous addresses (using a blacklist).